Spark plug for internal combustion engine

ABSTRACT

A spark plug having a tube-shaped, metal housing, an insulator which is retained by the housing, and an inner conductor arrangement embedded in the insulator, the inner conductor arrangement comprising one connection bolt, one burn-off resistor, one contact pin, and one platinum central electrode, and four ground electrodes, which are bent towards the central electrode and protrude beyond the insulator by the width of their thickness, being secured to the housing, the contact pin being coated and being shortened in its length such that the burn-off resistor is forward-positioned as far as possible towards the spark gap, and the platinum central electrode having a nail shape, whose rear part has a smaller diameter than its front part, which extends out of the insulator.

BACKGROUND OF THE INVENTION

The present invention relates to a spark plug for internal combustionengines.

BACKGROUND INFORMATION

A conventional spark plug is described in German Patent Application No.44 31 143. It proposes a spark plug which exhibits minimal wear. In thecase of the conventional spark plug, a rod-shaped inner conductor is setin a cylindrical metal tube which constitutes the housing, said innerconductor being enclosed in an insulator and having a current-limitingresistor in the electric circuit of the spark plug, saidcurrent-limiting resistor being arranged such that it isforward-positioned in the direction of the spark plug's spark gap asclose to the spark gap as possible. In addition, in this spark plug, thediameter of the inner conductor is reduced, as compared to conventionalspark plugs, as a result of which the capacitance of the spark plug isreduced. In this spark plug, in accordance with German PatentApplication No. 44 31 143, the electrode head of the inner conductor iscovered with a precious metal coating, and material having good thermalconductivity is used as the electrode head, which also acts as a heatbuffer.

SUMMARY OF THE INVENTION

In contrast to a conventional spark plug, a spark plug according to thepresent invention has the advantage of making available a greater volumethat is subject to wear by configuring the electrode in the form of a"nail head." In addition, by reducing the contact pin's diameter andlength, and by forward-positioning the burn-off resistor, one canachieve that fewer mechanical stresses caused by different thermalexpansion coefficients will occur. By reducing electrode burn-off, nicksin the ceramic can ultimately be substantially avoided. A furtheradvantage lies in the coating of the contact pin, through which means ahigh corrosion-resistance is achieved. Finally, by configuring theground electrode in a single or double bend and projecting it beyond theend face of the insulator, the electrical field is so configured thatceramic breakdowns are able to be advantageously prevented.

It is particularly advantageous to dope the platinum electrode. Thisdoping prevents the contact pin from corroding, since the platinumelectrode is sintered, gas-tight, into the ceramic. Through thegas-tight bonding of the electrode, the ceramic, and the contact pinusing an active solder, the contact pin is also prevented from corrodingand from retracting. Ultimately, an electrically conductiveceramic-metal mixture may be substituted for the contact pin, thecontact pin likewise being prevented from retracting, and mechanicalstresses being avoided as a result of having identical thermal expansioncoefficients.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a spark plug according to the present invention.

FIG. 2 shows the combustion-chamber-side end of the spark plug havingdifferently positioned ground electrodes, according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically shows the interior part of a spark plug in apartial cut-away representation. In this context, an insulator 11 isarranged in a metal, tube-shaped housing 10, the rotationally symmetricaxes of housing 10 and insulator 11 being coincident. Embedded ininsulator 11 is central electrode 12, as well as the individualcomponents described below for transmitting the voltage from connectionbolt 13 to central electrode 12. Inside insulator 11, connection bolt 13is joined to burn-off resistor 14. Burn-off resistor 14 is connected tocontact pin 15, so that the voltage is transmitted over contact pin 15to central electrode 12. Central electrode 12 is made of platinum andessentially has the shape of a nail. This means that the rear part ofplatinum central electrode 12 has a significantly smaller diameter thanthe combustion-chamber-side part of the platinum central electrode,which extends out of insulator 11. Contact pin 15 itself is covered witha corrosion-protection coating, e.g., with nickel or with anickel-silver alloy. In addition, compared with conventional sparkplugs, contact pin 15 is significantly shortened in its length andreduced in its diameter. By shortening contact pin 15, the burn-offresistor is lengthened and is forward-positioned in the direction of thespark gap, which has the advantages that have already been described inGerman Patent Application No. 44 31 143. Ground electrodes 16 aresecured to housing 10 and are bent towards the central electrode. FIG. 1shows that the ground electrodes are bent twice, provision being madehere for four ground electrodes. In this context, the ground electrodesecured to the housing is first bent in the direction of the centralelectrode and is then bent a second time in the axial direction, so thatthe end face of the ground electrode is facing in the axial direction ofthe spark plug. In this context, the end face of ground electrode 16protrudes by a specifiable amount beyond the end face of the insulator.The amount of the protrusion here corresponds roughly to the thicknessof a ground electrode.

FIG. 2 shows the spark plug having essentially the identical design asin FIG. 1. The sole difference from FIG. 1 lies in the positioning ofthe ground electrodes. In FIG. 2, the ground electrodes are bent once,thus making the end face of the ground electrode point in the radialdirection of the spark plug. But here also the ground electrodesprotrude beyond the front end of the insulator roughly by theirthickness. In this way, a favorable formation for the electric field isattained, thus preventing ceramic breakdowns.

Furthermore, the platinum central electrode can be additionally coated,e.g., using boronization, aluminization, nitration, or siliconization.This coating enables the platinum electrodes to be sintered, gas-tight,in the ceramic. The soldered connection of the platinum centralelectrode, the insulator, and the contact pin also contributes to agas-tight bond, thus preventing contact pin corrosion and retraction.Ultimately, the contact pin can also be replaced by an electricallyconductive ceramic-metal coating.

What is claimed is:
 1. A spark plug comprising:a tube-shaped metalhousing; an insulator embedded in the housing; a rod-shaped innerconductor arrangement disposed in the insulator, the inner conductorarrangement includinga connection bolt, a current-limiting resistorbeing forward-positioned towards a spark gap, a coated contact pin, anda nail-shaped platinum central electrode having a first part and asecond part, the first part being embedded in the insulator and having asmaller diameter than the second part, the second part protruding out ofthe insulator; and a plurality of ground electrodes being secured to thehousing and being bent towards the central electrode, the plurality ofground electrodes protruding beyond an end face of the insulator, anamount of protrusion corresponding to a thickness of the centralelectrode.
 2. The spark plug according to claim 1, wherein the pluralityof ground electrodes have a first bend and a second bend, the first bendbeing in a direction of the insulator and the second bend being awayfrom the insulator, so that an end face of the plurality of groundelectrodes points away from the housing.
 3. The spark plug according toclaim 1, wherein the plurality of ground electrodes include four groundelectrodes.